Stanley Gibbons has quickly begun recognising the rarity of this value on Birds III labels and, the last time I checked, a set of 6 mint labels was being valued at £60 by their GB catalogues and it's difficult to see these not continuing to improve in value.

As I write this post this commercially used example on cover is available:

The Stamp features the Greylag Goose and the cover was addressed to Greece. Unusually for modern commercial mail in Britain, the stamp is superbly used by an Edinburgh circular machine cancel. The cover may not be available for long but, at the time of writing, it was still available among these seller listings.

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